Studies on test rats have shown that even repeated, extremely high doses of theanine cause little to no harmful psychological or physical effects.[7]

L-theanine may help the body's immune system response when fighting infection by boosting the disease-fighting capacity of gamma delta T cells. The study, published in 2003 by the Brigham and Women's Hospital, included a four-week trial with 11 coffee drinkers and 10 tea drinkers, who consumed 600 milliliters of coffee or black tea daily. Blood sample analysis found that the production of anti-bacterial proteins was up to five times higher in the tea-drinkers, an indicator of a stronger immune response.[8]

Recently, marketing campaigns from tea companies have been emphasizing the natural presence of L-theanine in their tea and that L-theanine boosts the production of alpha brainwaves. [9]